II 



INSECTS AND PLANTS 



LET us consider, for a moment, what effect insects have 

 upon plants. Their action may be direct or indirect ; in the 

 former case insects may do actual damage to plants, or they 

 may cause definite diseases to arise as a result of their 

 injuries; in the latter case they may introduce fungoid 

 diseases. 



In the case of direct injury, various parts of the plant 

 may be destroyed, either in the process of feeding, nest- 

 building, or oviposition. The loss of an organ is not such 

 a serious matter to a plant as to an animal, nevertheless, 

 growth is seriously impaired when large numbers of insects 

 devour the leaves, which carry on carbon assimilation. 

 Other insects, such as grain beetles and moths, destroy 

 seeds and therefore impair reproduction, and this function 

 may also be impaired, at an earlier stage, by insects eating 

 the pollen. Others, also, interfere with the reproductive 

 parts of plants, but are mainly noxious, because they damage 

 parts of more use to man than to the plants concerned ; as 

 examples we may mention the damage to apples by the 

 codling moth and to cotton bolls by the cotton-boll weevil. 



Insects also cause direct injury in other ways, by ab- 

 stracting from the plant the liquid food so necessary for 

 its nourishment, or by removing so much material from 

 root or stem as to interfere with the plant's stability. 



Various physiological troubles may ensue as a result of 

 injury. Roots or stems may be so damaged that the plant 

 cannot obtain water, and wilting then takes place, or the 



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